The present invention relates to a multimode fibre having a refractive index profile, and also to a method of manufacturing the same.
From International patent application WO 00/50936 there is known a multimode fibre and a method of manufacturing the same, which multimode fibre has a core area of 62.5 xcexcm and a non-linear DMD profile which, if measured at a wavelength of 1300 nm, comprises a first average slope, measured over a first region, of (r/a)2=0.0-0.25, and a second average slope, measured over a second region, from (r/a)2=0.25 to 0.50, in which the first average slope is greater than the second average slope. The multimode fibre that is known from said document has been manufactured by means of Outside Vapor Deposition (OVD). Further details on pulse splitting are not given in said document.
The multimode fibre as referred to in the introduction is known per se from U.S. Pat. No. 4,793,843. Said document describes a method of manufacturing a preform, from which preform optical fibres are drawn, in which the preform has a refractive index profile which exhibits hardly any disturbance. Upon manufacture of such a preform, the hollow substrate tube is heated along the entire length thereof, with the heating element being reciprocated at a uniform speed along the length of the rotating substrate tube, after glass layers have been deposited on the inner surface. Under the influence of the surface tension of the softened substrate tube material, the diameter of the substrate tube gradually decreases until the substrate tube closes entirely, thus providing a preform in the form of a massive bar, from which an optical fibre is a drawn by heating one end thereof. A dopant which is generally used in the manufacture of optical fibres from quartz glass is germanium dioxide (GeO2). When a hollow substrate tube containing quartz glass doped with a slightly volatile dopant, for example germanium dioxide, is subjected to contraction, part of the dopant may evaporate from the glass layers that have been deposited last. This results in a disturbance in the refractive index profile, which disturbance has an adverse effect on the bandwidth of the optical fibre. According to said U.S. patent specification it has appeared that the so-called dip in the refractive index profile of optical fibres can be prevented if, at the moment of closing of the hollow substrate tube, a gaseous etchant is passed through the substrate tube, which is still hollow at that point, so as to obtain a massive bar.
For the new generation of optical fibres, the transmission of large amounts of data is of major importance, and it is desirable to manufacture multimode fibres for so-called 10-gigabit or  greater than 2.5 Gb Ethernet systems which have a refractive index profile which is free from disturbances. Such special profile disturbances in a multimode fibre lead to an undesirable disturbance of the light passing through the fibre.
The object of the present invention is to provide a multimode fibre which is free from disturbances in the refractive index profile thereof.
According to the present invention, the multimode fibre as referred to in the introduction is characterized in that the area surrounding the centre of the fibre has a refractive index profile such that the responses of a DMD (Differential Mode Delay) measurement carried out on a fibre having a length of at least 300 m are obtained without any pulse splitting occurring in the centre of the fibre.
Experiments carried out by the present inventors have shown that the centre of the fibre is of major importance as regards the suitability of a multimode fibre for high-speed data transmission. The suitability of a multimode fibre for high-speed data transmission is evaluated by means of a so-called DMD (Differential Mode Delay) measurement, in which the transmission of a light pulse is measurement at several radial positions through the core of the optical fibre. A fibre length of 300 m is a length which is generally used as the maximum length in practice. Moreover, the amount of fibre waste will be small when a DMD measurement is carried out on a fibre of such length.
It is preferred to carry out the DMD measurement by means of a laser at a wavelength of 850 nm and a short pulse duration and a small spectral width, in which a so-called single mode fibre is used for irradiating the multimode fibre to be measured. If a multimode fibre exhibits a small deviation in the central part of the ideal refractive index profile, pulse widening or a so-called double pulse in the DMD curve will be observed. As a result of said pulse widening, light will pass through that part of the core of the fibre at varying speeds, which is undesirable.
It is desirable for the multimode fibre that is used in the present invention to have a refractive index profile in accordance with the following equation:
n(r)=n1[1xe2x88x922xcex94(r/a)xcex1]1/2 
wherein:
n1=the refractive index value of the centre of the fibre
r=the radial position in the fibre (xcexcm)
xcex94=the refractive index contrast of the fibre
xcex1=the profile shape parameter
a=the radius of the fibre (xcexcm)
The profile shape parameter xcex1 determines the wavelength at which the maximum bandwidth of the fibre is measured. In the case of a higher xcex1-value, the maximum bandwidth is measured at a lower wavelength. Changing the xcex1-value in this manner makes it possible to optimise the bandwidth of a fibre at a particular wavelength. For the next generation of optical fibres, in particular fibres having a radius of more than 20 xcexcm, the profile shape parameter xcex1 is changed so as to optimise the bandwidth of the fibre at a wavelength of 850 nm. A very precise xcex1-profile can be obtained by depositing layers having a precisely defined refractive index value on the interior of the hollow substrate tube. The present multimode fibre preferably has a core having a diameter of 50 xcexcm.
The present invention further relates to a method of manufacturing a multimode fibre as defined in the appended claims.
The method for manufacturing the present multimode fibre is known per se from Dutch patent NL 1013944 to the present applicant, whose contents can be considered to be fully incorporated therein. The method that is known from said patent relates to a method of depositing glass layers, which may or may not be doped, on the interior of a substrate tube by means of a chemical vapour deposition (CVD) technique using a reactive gas mixture, so as to obtain a preform having a precisely define refractive index profile, which method comprises the steps of:
a) determining the desired refractive index profile of the preform to be manufactured,
b) precisely adjusting both the composition and the supply rate of the reactive gas mixture for manufacturing the intended preform in conformity with the refractive index profile determined in step a),
c) introducing the reactive gas mixture, under the conditions adjusted in step b), into the substrate tube and effecting a reaction therein so as to achieve deposition of the glass-forming oxides onto the interior of the substrate tube,
d) contracting the substrate tube obtained from the deposition process of step c) into a preform and subsequently subjecting said preform to a refractive index profile measurement,
e) comparing the refractive index profile determined in step a) with the refractive index profile measured in step d), and
f) correcting the differences in refractive index profiles measured in step e) by adapting the composition of the reactive gas mixture as a function of time during the subsequent deposition process.
Using the aforesaid steps a)-f), it is possible to obtain precisely defined refractive index profiles in a preform, in which there is feedback of the refractive index profile measurement in the final preform and the gas dosage used in the deposition process. The results of the refractive index profile measurement as obtained in step d) are used for adjusting the production process in step c) for manufacturing the preform. On the basis of the refractive index profile measurement obtained in step d) it is thus possible according to the aforesaid invention to determine the extent to which the process, in particular the composition of the reactive gas mixture, is to be adjusted in order to approach the desired refractive index profile of step a) as nearly as possible. After both the composition and the supply rate of the reactive gas mixture have been precisely adjusted in accordance with step b) of the aforesaid invention, the deposition process according to step c) is carried out. Once the deposition process according to step c) has been terminated, the preform thus obtained is subjected to a refractive index profile measurement in step d). The refractive index profile measured in step d) is then compared with the refractive index profile determined in step a), after which a correction of the differences in the refractive index profiles measured in step e) may take place in step f) by adapting the composition of the reactive gas mixture thereto as a function of time during the subsequent deposition process. It should be understood that the correction carried out in step f) may require that the composition of the reactive gas mixture be adapted continuously during the subsequent deposition process. If the differences in refractive index profiles that are measured in step e) are acceptable within specific tolerances, however, no correction of the composition of the reactive gas mixture will take place during the subsequent deposition process. Correction will only take place when the differences measured in step e) exceed a specific tolerance range.
The PCVD process as described above makes it possible to manufacture a multimode fibre having a refractive index profile, in which the core is composed of several thousand layers. The refractive index value can be changed for each of said layers, which results in a very precise xcex1-profile.
In developing the next generation of fibres, the present inventors have found that the area around the centre of a fibre, which has a diameter of 1-6 xcexcm, is of major importance in preventing the occurrence of pulse splitting in the centre of a fibre when carrying out a DMD measurement, which measurement takes place at a wavelength of 850 nm with an irradiation MFD (Mode Field Diameter) of  less than 6 xcexcm.